I take it that you want to UPLOAD an image into the gallery and for your avatar. Here's the dummy's

guide to do it:
In the gallery:
1) Open the gallery from the left hand panel of the website
2) Click on the gallery you want to upload an image to. e.g uncategorized. This will open a page with various thumbnails of images
3) Click on the link that says "New Image" on top or bottom of the thumbnails
4) Click on the button that says "Choose File" and browse to the folder where your screenshots are saved. Most likely they should be in Dcouments/The Lord of the Rings Online folder
5) Select the image you want to upload and press ok.
6) Give a caption to the image and click save.
Image for the avatar:
1) Click on the settings link on the top right corner. This will open personal settings page.
2) Click on the tab link that says avatar
3) Click on "Choose File" button and browse to the image on the hard drive which you want to set as an avatar.
4) click Ok and then click update to upload the avatar image
Hope this helps